23 October 2017
Dark Matter is a puzzle that has plagued scientists for almost a century. Here’s everything we know, and everything we don’t.
Planets, stars, asteroids, galaxies – the things that we can actually see – constitute less than 5% of the total universe. So what makes up the rest? Therein lies the mystery.
Dark matter is the name we give to all the mass in the universe that remains invisible, and there’s a whole lot of it. Research suggests that about 70% of the universe is composed of dark energy, whilst the remaining 25% is composed of a mysterious substance known as dark matter. We can’t see it, we don’t understand it, but we know it’s out there.
Dark matter is really hard to study, because we have no way of seeing it. This weird substance doesn’t interact with light, so it’s totally invisible.
So how do we know it exists? Well, because it affects the universe in strange ways; and that’s something we can see.
So dark matter can’t be seen, but it can be felt because of its powerful impact on space.
Dark matter exerts ‘gravitational force’, meaning that it draws other matter towards it. And there’s so much dark matter that its gravitational force is enough to hold entire galaxies – like our own Milky Way – together. That’s why dark matter is often likened to a giant spider’s web, meshing galaxies in place.
We can also see the effects of dark matter simply by looking up at the sky. When astronomers observe distant galaxies, they often appear stretched and oddly shaped.
This effect is known as ‘gravitational lensing, and it’s caused by dark matter’s gravitational force. This force is so huge that it physically bends the light around galaxies, distorting their appearance.
Scientists have been able to piece together maps of space, indicating where they think dark matter is hiding.
By studying ‘relic radiation’ left over from the Big Bang, scientists can identify areas in space where more radiation exists – and more radiation means more matter. In this way, we can identify ‘hotspots’, where higher levels of dark matter may be concentrated.
The majority of scientists believe that dark matter is some sort of exotic particle.
We already know about photons, electrons, quarks and many other particles, but there may be plenty of others waiting to be discovered. One or more of these unidentified particles could be responsible for effects – like gravitational lensing and the ‘spider’s web’ – that we associate with dark matter.
Dark matter might be an undiscovered particle, or it might be nothing at all. Some scientists believe that the effects we associate with dark matter are actually caused by gravity.
It’s possible that our existing theory of gravity is flawed, and that the effects we attribute to dark matter could simply be a quirk of gravity that we don’t yet understand.
So is dark matter a particle or a consequence of gravity? Well, we don’t know, and more research is needed before either theory can be proved true or false.
If dark matter particles exist, then they should occasionally collide with one another: an interaction that produces radiation. And so spacecraft have been fitted with advanced detectors to search for signs of this radiation. Some interesting results have cropped up, but the search for hard evidence is still on.
If hard evidence for dark matter is ever found, it will be in the form of very small, subtle effects. And so, in order to detect these tiny signals, scientists have developed dark matter laboratories deep underground, far from the effects of overpowering background noise.
In the UK, there’s Boulby Underground Laboratory: a dark matter lab in North Yorkshire, located over 1,000 metres below the Earth’s surface. And other similar facilities exist across the globe, from Canada to Australia.
There’s still a whole lot we don’t know when it comes to dark matter, and some mysteries might never be unravelled. But as technology advances, we are edging closer towards the truth.
Dark matter may be complex, elusive and mysterious, but scientists all over the world will continue the hunt – and what they find could eventually transform our very notion of the cosmos.
Last updated: 13 April 2018